In order to remove coatings from underlying surfaces, industry is moving away from the use of chemical striping agents and towards the use of blasting techniques. With these blasting techniques, abrasive or non-abrasive media or water are blasted onto the coated surface at high velocity to remove the coating. There is a wide variety of blasting nozzles and blasting media on the market. The most widely used blasting systems use pressurized blasting media. Other systems use a suction feed in which the blasting media is fed into a high velocity air stream via suction. Suction feed systems do not typically have as much power as a pressurized blasting media system. Commonly used blasting media includes sand, plastic, glass or water, but there is also a wide range of specialized blasting media ranging from steel shots, on one hand, to corn starch or soybean media on the other.
In a typical set up, the user holds the blasting nozzle by hand and blasts the media towards the workpiece. The distance that the blasting nozzle is from the workpiece is commonly referred to in the trade as “stand-off” distance. The stand-off distance is important because it regulates the velocity of the blasting media as it impacts the coated surface. The user aims the high-velocity jet containing the entrained blasting media at the surface until the coating is removed at that spot. The user then moves the jet across the surface in a back and forth motion in order to remove the coating from the surface. When the user starts a job, the user might not know the thickness of the coating and therefore must guess through trial and error or experience as to the appropriate stand-off distance.
If the nozzle is too close, impact of the blasting media may damage the surface. On the other hand, if the stand-off distance is too great, the blasting media will not have enough velocity to remove the coating. The ideal stand-off distance for a majority of blasting media is about 12 inches. Corn starch or soybean media or water blasting requires a distance of about 6 inches. On the other hand using steel shots as a blasting media may require a stand off distance of about 18 inches. The appropriate stand-off distance for a given situation also depends on the pressure at the blasting nozzle as well as the thickness of the coating and the characteristics of the underlying substrate. For example, when the underlying substrate is made of a certain types of light weight composite material, holding the blasting nozzle too close to the substrate might not only damage the surface of the substrate, but might actually blow a hole through the substrate.
Thus, the optimum stand-off distance varies in the field depending on many factors including the type of blasting system being used (e.g. pressurized blasting media, water blasting, suction feed, etc.) and its set up parameters, the type of blasting media being used, the nature of the underlying substrate, the nature of the coating and possibly other factors. While there is some published data on what is believed to be the optimum stand-off distance under various conditions, such information is not often readily available to the user. Moreover, even armed with knowledge of the optimum stand-off distances under various conditions, it is difficult for users to maintain the blasting nozzle at the optimum stand-off distance from the surface as they move the nozzle back and forth the remove the coating. This can be especially difficult for novices.
The Assignee of this application has developed optical targeting and positioning systems for spray painting apparatus. Representative systems are shown in Klein II et al U.S. Pat. No. 5,598,972 issued Feb. 4, 1007; Klein II et al U.S. Pat. No. 5,857,625 issued Jan. 12, 1999 the disclosures of which are hereby incorporated by reference. Generally, these patents illustrate the concept of mounting a light beam emission arrangement on a spray paint gun or within the housing of the spray paint gun. The light beam emission apparatus directs a pair of light beams in a direction from the gun towards the surface to be sprayed. The light beams are oriented so as to converge towards each other as the beams propagate in a direction away from the gun towards the surface. The light beams form spots on the surface. The spots are aligned on the surface, such as merged together to form a single point of light on the surface, when the spray head of the spray gun is held at a predetermined stand-off distance from the surface. The angle of the light beams can be adjusted to vary the convergence distance, thus allowing the user to customize the desired stand-off distance indicated by the optical positioning system. The user can accommodate different spray painting operating parameters or characteristics, such as air pressure, coating type and the like, when setting up the optical positioning system for the appropriate stand-off distance, thus facilitating optimal application of the spray coating (e.g. paint) to the surface and minimizing overspray and waste. While this type of targeting and positioning system has been proven effective to optimize the application of sprayed coatings, it has not heretofore been used in connection with coating removal systems using blasting media.
Horan U.S. Patent Application US2003/0178503A1 entitled “Single Beam Spray Gun Positioning System”, filed on Mar. 20, 2002 and published on Sep. 25, 2003 discloses a targeting and positioning system for a spray paint gun in which a single light beam is used to provide a rough estimate the distance of the spray nozzle from the surface being painted. The system does this by illuminating an optical beam at an angular orientation with respect to the center line of the spray pattern. It then requires the user to align the illuminated spot on the surface with the approximate center of the spray pattern on the surface being painted. As with the two beam systems described above, the desired stand-off distance can be adjusted by adjusting the direction of the light beam.